We are investigating the role of tumor suppressor genes in the pathogenesis of human cancer. Our recent findings are as follows: 1) We have studied the functional properties of mutant retinoblastoma (Rb) genes isolated from human tumor samples. Our analyses of the mutant RB protein expressed in these tumors reveals a pattern of complete inactivation in each of these samples including loss of ability to suppress growth of RB(-) lung cancer cells. 2) We have established the molecular basis of incomplete penetrance of familial retinoblastoma and the phenotype of spontaneous tumor regressions seen in these kindreds. We propose that this may be a general mechanism for incomplete penetrance and/or spontaneous tumor regression seen in other malignancies as well. 3) We have generated a series of in vitro point mutants at selected regions of the RB protein to delineate the functional domains of the RB that confer tumor suppressor activity. 4) We are studying the interaction of the RB protein with a family of cellular binding proteins that are believed to modulate RB tumor suppressor activity. We have obtained full- length cDNA clones for the retinoblastoma binding protein-1 and binding protein-2, and we have successfully raised specific antisera directed against these products. We are defining the functional role of these important binding cellular products and determining whether they also are targets for mutations in human cancer. 5) We have cloned and characterized a novel member of the human HSP70 protein chaperone family. Since a major function of these protein chaperone molecules is to bind to mutant (misfolded) tumor suppressor proteins we are studying the pattern its pattern of peptide binding. 6) We have established that expression of the retinoblastoma protein in RB(-) lung cancer cells results in suppression of tumorigenicity.